Right this was the process I used on my first attempt. And I used Luma Key. (it will be a different process for blown highlights)
First off looking at the original source with crushed blacks the Empire Strikes Back Blu-Ray not sure if this is the best example as sometimes elements get moved around. anyway make 3 video tracks and put this on track 2.
Now we take the alternate source that has the shadow detail present this goes on track 1.
We lay this on top of the image that we want to combine but in order to line the 2 images up we need to first make the image with the shadow detail opaque. We Add a 2D-DVE to this Re-size and scale as needed and lay over the top so that if possible it matches exactly using the highlights as a guide and matching the geometry of the 2 images. (If I have a slightly different frame or something I apologize but for the purposes of explaining how to do this as I don’t have much time today due to other comittments). I sharpened the source with shadow detail as it was a lot more blurred than the source I was trying to blend.
Now once we have the 2 images matched we move the image or clip with the shadow detail on to track 3 keeping the geometry matches but now you must make the image or clip have 0% opacity so it is now solid.
We now go back to the image or clip that has shadow detail missing.
Here we apply a Luma Key to the image or clip. Generally for shadow detail you will have a high tolerance and little edge softness. Here was the result.
Now you have shadow detail the image will then need to be matched to the rest of your footage so contrast and color so on and cropping or masking around the affected area with no shadow detail.
If you want me to try this on ghostbusters or nightmare on elm street then it would be beneficial to show me the shots or comparison sources to restore shadow.
I won’t be touching Adywans edit but I think it’s useful for everyone to try to deal with this problem because it’s not unique to any film but a plethora of titles. And I need to try this technique on other sources.